The invention described herein is generally related to polyacetylenic and polyaromatic polymers, particularly those suitable for doping to produce electrically conductive polymers.
Certain polyacetylenic and polyaromatic polymers have been of considerable interest since the recent discovery that the electrical conductivity of such polymers can be significantly increased by doping them with various dopants, as described for example in the U.S. Pat. No. 4,321,114 to MacDiarmid et al. In some cases, the doped polymers exhibit electrical conductivities comparable to that of metals. Although polyacetylene is the most studied conductive polymer, the polyaromatics such as polyparaphenylene are also of interest because it is known that they can in some cases also be made electrically conducting by doping.
Polyacetylene occurs in the cis- and trans-isomeric forms, which are represented as follows: ##STR1## The cis-isomer is generally stable only at low temperatures and can be converted to the trans-isomer (the thermodynamically more stable form) by heating at 150.degree.-180.degree. C. for 30 minutes to 1 hour.
As discussed further below, polyacetylene is generally formed as thin plastic film. Electron microscopy studies show that the film consists of randomly oriented fibrils, the diameter of which depends on the reaction conditions under which the film was prepared. X-ray diffraction analyses indicate that the films are polycrystalline, with the principal interchain spacing being on the order of 4.39 Angstrom. Films in the cis-isomeric form are relatively flexible and can be stretched at room temperature up to three times their original length, with partial alignment of the fibrils resulting. Films in the trans-isomeric form are more brittle and can be stretched only to a very small extent.
All of the previously known polyacetylene films are insoluble in common solvents. They are also thermally unstable, decomposing rather than melting upon being heated. These characteristics represent the primary difference between the previously known polyacetylenes and the polyacetylene of the present invention, which is soluble in a number of common organic solvents and which can be melted without decomposing. This is considered to be an important difference, inasmuch as it renders possible the manufacture of electrically conductive polyacetylenes which are thermoplastic, and can thus be formed into various shapes by common thermoplastic forming techniques, and which are also soluble in common solvents, also greatly enhancing the utility of the material in forming various types of electrically conductive polymeric articles.
Polyacetylenes have been previously synthesized primarily by the use of what are known as Ziegler-Natta catalysts. Such catalysts are generally transition metal derivatives complexed or reduced by an organometallic compound. A primary example of such a catalyst is the mixture of triethylaluminum ((C.sub.2 H.sub.5).sub.3 Al) and tetra-n-butoxy titanium ((n--C.sub.4 H.sub.9 O).sub.4 Ti). A polyacetylene film may be produced by wetting the interior walls of a glass reaction vessel with a toluene solution of this catalyst and then admitting gaseous acetylene at any pressure between a few centimeters Hg and one atmosphere. The gaseous acetylene is polymerized and deposited as a film on the surface of the wetted vessel walls over a period of a few seconds to an hour, depending on the acetylene pressure and the temperature. The film can be washed, dried, and readily peeled from the surface of the vessel. The film can thus be removed as a free-standing film or can be left in place on the substrate surface. The thickness of the film can be varied from 10.sup.-5 cm up to approximately 0.5 cm depending on the reaction conditions. If polymerization is conducted at a temperature below -78.degree. C., the film is formed predominantly in the cis-isomeric form. If polymerization is conducted at room temperature, a mixture consisting of 60% cis- and 40% trans-isomeric form is obtained. At a temperature of 150.degree. C. (using a decane solvent), the trans-isomer is formed exclusively. As noted above, the cis-isomer can be readily converted to the trans-isomer simply by heating.
The electrical conductivity of undoped polyacetylene films depends on the cis-trans content of the film, ranging from 10.sup.-5 (ohm-cm).sup.-1 for the trans-isomer to 10.sup.-9 (ohm-cm).sup.-1 for the cis-isomer. The more conductive trans-isomer is considered to be a semiconductor, as conductivities in the 10.sup.-5 to 10.sup.-6 (ohm-cm).sup.-1 range are generally considered to be in the semiconductor range. The band gap of the trans-isomer is approximately 1.4 electron volts, which is also similar to that of common semiconductors such as amorphous silicon, cadmium sulfide and zinc sulfide. However, the measured conductivities of the trans- and cis-isomers are believed to represent the conductivities of the polymers as contaminated with trace amounts of aluminum and titanium remaining from the catalyst used to induce polymerization. The conductivities of the pure polymers are believed to be considerably lower and have not been accurately determined due to the practical difficulty of obtaining uncontaminated polyacetylene films.
The conductivities of polyacetylene films can be significantly increased by doping them with various types of dopants. Doping of polyacetylene has been known to increase the electrical conductivity to as high as approximately 1000 (ohm-cm).sup.-1, which is comparable to the conductivity of metallic mercury. Either n- or p-type dopants can be utilized. Doping is typically achieved by exposing the film to a vapor or solution of the dopant. For example, polyacetylene films can be doped by exposure to the vapor of an electron-attracting (p-type) substance such as Br.sub.2, I.sub.2, AsF.sub.5, H.sub.2 SO.sub.4, or HClO.sub.4. Alternatively, the film can be doped with an electron-donating (n-type) dopant, for example by immersing the film in a solution of sodium naphthalide in tetrahydrofuran.
As already noted, the previously known polyacetylene and polyaromatics are generally insoluble and cannot be melted without decomposing. These characteristics have heretofore placed substantial limitations on the extent to which these materials can be processed to form useful, electrically conductive, nonmetallic articles.